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Bad Alternator symptoms

Bad Alternator symptoms

Five symptoms of a bad alternator

Your car’s alternator doesn’t last forever. It usually provides some clues before it bites the dust. Here are the top five alternator failure symptoms you should act on right away.

What an alternator does

1) Provide power to run the engine’s computer, ignition and fuel systems and the vehicle’s electrical accessories while the engine is running.
2) Recharge the battery for power lost from starting and from excess power that was drawn from the battery during periods that exceeded the alternator’s ability to provide that power at low engine RPMS.

NOTE: Although the alternator is designed to recharge the battery after excessive power draw events,  the alternator was never designed to recharge a completely dead battery. If you use your alternator to recharge a dead battery after a jump start, you’re shortening the life of your alternator.

If your battery is dead, recharge it with a real battery charger, not your alternator. Once you damage your alternator by using it as a battery charger, there’s no turning back. A new alternator costs around $495. A battery charger costs less than $100. So here are the symptoms of a bad alternator.

Bad Alternator symptoms #1—Battery or charging warning light is lit

When the charging or battery light is lit, chances are your alternator or the voltage regulator portion of the charging circuit has failed. The failure can be an electrical failure like failed diodes or mechanical like worn brushes or rotor or stator windings, or even a worn bearing. Before you start testing the alternator function, perform a full battery test and clean the battery posts and terminals to eliminate high resistance as a possible cause. If the battery doesn’t have a full charge, use a battery charger to bring it up to full charge. See this post for helpful information on battery terminal cleaning and alternator  testing.

Bad Alternator symptoms #2 —slow cranking when starting

Slow-cranking can be a sign of a severely discharged battery caused by an internal issue, parasitic current drain, high resistance at the battery posts and terminals and ground connection, or alternator failure. Slow-cranking can also be caused by an aging battery that has built up too much sulfation. Or, the slow cranking can be caused by a parasitic battery drain due to a computer module that fails to go into “sleep” mode, causing high current draw. See this post for information on battery drain. If you’re ruled out a damaged battery and parasitic current drain, then it’s likely the alternator has failed to keep up with the electrical demands while driving and the excess electrical demands depleted the battery.

Always start diagnosing a slow cranking condition by cleaning the battery posts and terminals and checking battery voltage to make sure it’s fully charged.

Bad Alternator symptoms #3—Dim headlights lights at idle

The alternator’s job is to produce all the power needed to run the vehicle’s electrical accessories. Well, headlights and dash lights only consume about 15-20-amps and that’s well within the alternator’s generating capabilities even at idle speed. So, if you’re at a stop sign and your headlights are dim, and you’re not running any other electrical accessories, you should suspect a bad alternator.  In other words, any working and properly sized alternator should be able to supply 15-amps at idle.

However, an alternator may not be able to supply enough power at idle to keep up with seat heaters, rear window defogger, heater blower on high AND headlights all running at the same time. If your headlights dim in that scenario, it’s not your alternator’s fault. So don’t confuse dim headlights with when no other electrical loads are on with dim headlights when everything else is running.

In fact, if you’re at a stop and running all those accessories, you’re actually discharging your battery at a high rate. Take a look at what these electrical accessories draw

Rear Window defogger 25-amps, Electric AIR Pump 25-amps, Heated Seats 5-amps per seat, Headlamps (high) 20-amps, Blower Motor (High) 20-amps, Headlamps (low) 15-amps, Brake Lights 6-amps, Windshield Wipers 6-amps, Ignition 6-amps

Let’s say it’s winter and you’re at a stoplight. You’re running your rear defogger, heated driver’s seat, wipers, headlights, blower on high and your foot is on the brake pedal. You’re consuming around 83-amps.
At an engine idle speed of around 750-RPMS, your alternator can only output about 35% of its rated output. So your 165-amp alternator can only provide around 58-amps, resulting in a 25-amp draw from the battery and a voltage drop that will dim your headlights. In this case, dim headlights would be considered NORMAL

Bad Alternator symptoms #4—Grinding or whining noise

Alternators are driven by a belt. If the belt isn’t at the proper tension or the belt ribs are worn, it will slip around the alternator pulley, making a grinding, whining, or chirping noise. Alternators also contain a front and rear bearing that can wear out prematurely from excessive heat or tension. A grinding noise is often the sign of a failing bearing. To check for bearing noise, follow the tips in this post.

Bad Alternator symptoms #5—Burning smell

As alternator bearings fail they produce resistance and that resistance can cause the drive belt to slip, overheat and burn, causing in a burnt rubber smell. An overheated alternator can also deteriorate the insulation on the stator and rotor windings, which also causes a burning smell.

©, 2019 Rick Muscoplat


Posted on by Rick Muscoplat

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